As a liquid developer, generally, liquid developers in which colored resin particles (hereinafter, also referred to as toner particles) containing a coloring agent such as a pigment are dispersed in an electrical insulating medium are used. Examples of a method of producing such a liquid developer include a coacervation method. The coacervation method is a method in which a solvent is removed from a mixed liquid of a solvent in which a resin is dissolved and an electrical insulating medium in which a resin is not dissolved, and thereby the resin contained in the mixed liquid in a dissolved state is precipitated so as to encapsulate a coloring agent to form colored resin particles, and the colored resin particles are dispersed in the electrical insulating medium.
The liquid developers obtained by such a method are considered to improve in an electrophoretic property because the shape of the colored resin particle is almost spherical and the particle size of the colored resin particles is uniform.
However, the coacervation method has a problem that the colored resin particles tend to agglomerate during the precipitation of the resin and therefore the dispersion stability and the optical properties of the developer to be obtained are inadequate due to an increase in particle size.
Therefore, in order to solve the problem, in the coacervation method, a method, in which the colored resin particles are dispersed stably in the electrical insulating medium by adsorbing one of a polymer compound having an acid group and a polymer compound having a basic group on the surface of the coloring agent and further encapsulating the resulting coloring agent adsorbing one polymer compound in the other polymer compound, has been proposed (for example, see Patent Document 1).
However, the above-mentioned method itself is a technology based on a hitherto well known concept of acid-base interaction in which dispersion is stabilized by treating the surface of the pigment with a compound having one of an acid group and a basic group to enhance an affinity for a resin having the other group. In addition, disclosed compounds are merely substances very commonly used such as an acrylic resin, a styrene-acrylic resin and the like, and effects of improving adequately the dispersibility of fine colored resin particles cannot be expected. Further, there is a problem that these compounds cause a reduction in an insulating property or deterioration in a charging characteristic and an electrophoretic property of the colored resin particles, and a good image quality cannot be attained in terms of suitability for the liquid developer.
Patent Document 1: Japanese Kokai Publication 2001-31900